1. Field of the Invention.
This invention relates to vacuum lifting systems. More particularly, the invention relates to an automatic sensing valve useful in multi-port vacuum pads for lifting operations involving loads of varying configurations and surface characteristics; in such use, the valve of my invention automatically blocks all ports except those ports resting on suitable surface areas, so that vacuum efficiency is maximized.
2. Description of the Prior Art.
Vacuum lifting systems are in common use as effective means to lift and/or position loads having "vacuum-friendly" surface characteristics--i.e. surfaces to which a vacuum pad will adhere under vacuum. Such systems normally include vacuum generating means such as a pump, often with an interconnected evacuated reservoir tank to sustain vacuum in the event of pump failure, one or more lifting pads so constructed as to form a vacuum seal with suitable load surfaces when vacuum is applied, and suitable vacuum conduit means, such as hoses, interconnecting the pad or pads with the vacuum pump. Each pad is suspended by one or more chains or the like from either a crane hook or a frame on which the vacuum pump is mounted, the frame in turn being suspended from a crane hook. In use each vacuum pad is lowered into position on the load surface, the vacuum pump is activated, and when a vacuum seal has been established between the pad and the load surface, the assembly is lifted. When the lifting operation is completed, the vacuum is released, either by means which simply allow atmospheric air to flow to the pad or, if more rapid release is desired, by means which actually blow air through the conduit and pad; in either event, when the vacuum is released, the pad and other parts of the assembly can be lifted away from the work.
As is known to those skilled in the art, the lifting capacity of a vacuum system is determined by several factors, including the capacity of the vacuum pump, number, size and configuration of vacuum pads, etc. The rate at which the design vacuum level is reached, which can be very important in high volume or high speed lifting operations, is determined by many of the same factors as capacity, but is also related to the rate of air flow permitted by the vacuum hoses, lifting pad orifices etc.; the higher the flow rate, the more rapidly design vacuum level can be reached. Vacuum pads of the prior art vary in size and shape (e.g. oval, square etc.) according to the type of load with which they are to be used. Additionally, most such pads have heretofore included a resilient sealing member around the periphery of their working surface--i.e. the surface facing the load surface--and a single orifice or port through which the working surface communicates with the hose to the vacuum pump.
New applications for vacuum lifting continue to arise as the effectiveness of the method is appreciated. One such application involves lifting in one operation a group of finished wooden floor boards or the like. In this situation a number of adjacent boards having typical widths of 2 to 3 inches and lengths of 8 to 12 feet are finished, e.g. by application of varnish, in one operation and emerge from the finishing equipment at one time, whereupon it is desired to lift the entire group as a unit and move them to a storage location. Although the finished surface of each such board is suitable for vacuum lifting, the unavoidable interstices between adjacent boards allows the flow of air, which prevents the formation of a vacuum seal if use is attempted of one of the above described prior art vacuum pads shaped to span the group of boards. Further compounding the problem is the fact that the group of boards to be lifted may vary in number and thus in combined width; hence, a prior art vacuum pad wide enough to span a particular number or width of boards will be unusable for lifting groups of fewer or narrower boards due to air leakage at the sides of the load.
Another lifting application where prior art vacuum pads cannot normally be used is one involving a load with varying surface characteristics, wherein some portions of the surface are suitable for forming a vacuum seal, while adjacent portions are not, for example because they contain depressions or holes.
One approach to overcoming problems such as those discussed has been to provide a vacuum pad in which the working face defines a plurality of spaced vacuum ports and a sealing member covering the entire working face includes holes corresponding to the ports, each port communicating with the vacuum hose through a manifold arrangement. In such prior art pads, a normally closed valve is mounted at each port and a sensing finger linked to the valve extends through the port and beyond the sealing member. If a sensing finger in a particular port is displaced by the load surface when the pad is brought in contact with the load, it causes the valve to open and thereby allows the vacuum pump to operate on that port with the intent of forming a vacuum seal at that part of the work surface.
Theoretically, with the just described multi-port vacuum pad, the vacuum pump will operate only on those ports in the working face where the sealing member is in contact with suitable load surface areas. However, two principal problems still exist with such a pad. First, the use and installation of the fingeroperated sensing valves adds significant expense and complexity to the pad. Second, the sensing fingers can be displaced by an isolated high point on the load surface or by the edge of a load adjacent to an open space; in either case, no seal is formed at such ports when the valve opens, and lifting either does not occur or is highly inefficient and unsafe because only a partially effective seal is formed between the vacuum pad and the load.